feat(T03): OCR-Erfassung via OpenRouter Qwen2.5-VL + OCR UI with drag-and-drop

backend/app/tasks/ocr_processing.py

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+"""Async OCR processing task using background tasks.
+
+In production this would use Redis Queue (RQ) or Celery.
+For now, we use FastAPI BackgroundTasks to trigger async processing.
+"""
+
+from __future__ import annotations
+
+import logging
+import uuid
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.database import async_session_factory
+from app.services.ocr_service import process_ocr
+
+logger = logging.getLogger(__name__)
+
+
+async def run_ocr_processing(result_id: uuid.UUID) -> None:
+    """Background task: process OCR result asynchronously.
+
+    Creates its own DB session (independent of the request session)
+    so the HTTP response can return immediately.
+    """
+    async with async_session_factory() as session:
+        try:
+            await process_ocr(session, result_id)
+            await session.commit()
+            logger.info("OCR processing completed for result %s", result_id)
+        except Exception as exc:
+            await session.rollback()
+            logger.error("OCR background task failed for %s: %s", result_id, exc)
+            raise
+        finally:
+            await session.close()